Due to the stability of the resonant frequency of the piezoelectric material, a piezoelectric resonator is commonly used as a frequency source in electronic circuits.
FIGS. 1 and 2 show a typical construction of a resonator comprising a piezoelectric material of appropriate shape, such as a resonator plate 6, fixed by an internal fastening structure 5 to the interior of an enclosure or hermetically sealed package 1. The enclosure may be filled with gas, usually inert gas, or house a vacuum.
One recognized problem in art of resonators relates to a parameter called acceleration sensitivity. For applications that employ the piezoelectric resonator under varying acceleration conditions, the reliability of the piezoelectric resonator as a stable frequency source is jeopardized.
Additional problems are faced when a resonator is exposed to G-shock. Acceleration sensitivity is related to resonant frequency variations resulting from external vibrations, but G-shock is a pulse inertial load in nature due to high external acceleration acting upon for a short time. The effect of G-shock on a resonator is what is known as a “residual frequency shift”, which is described in detail below with reference to the “elastic recovery time” definition.
In a worst case G-shock can cause non-operation due to resonator breakage or fastening structure breakage. In some applications, the level of G-shock might induce inertial load high enough to cause elements of the fastening structure to undergo plastic deformations resulting in permanent, “residual frequency shifts” in the piezoelectric resonating plate due to changes in strain/stress conditions. Also, changes in resonating plate position relative to the enclosure will influence static capacitance, which also results in residual frequency shifts.
Some of the terminology used to describe the factors that affect resonator frequency will now be explained.
As used herein the term “electromechanical deformations” relates to a deformation that occurs when an active area of the resonating plate experiences periodic deformations due to the piezoelectric effect induced by an applied periodic electrical field.
As used herein the term “mechanical strength” describes the ability of the fastening structure to hold the resonator plate with minimal displacement of the resonating plate relative to the enclosure. If joints are too flexible, the resonator plate will move as a rigid body inside the enclosure under external accelerations. Ideally, the resonator plate does not move relative to enclosure, i.e. the enclosure and resonator plate inside move as a single body and there are no strain/stresses induced by deformations of internal fastening structure. The acceleration sensitivity of the resonator is then reduced to only variations in electromechanical deformations in a gravitational field (g-sensitivity effect).
As used herein the term “thermal strain” is the result of different thermal deformations of joined parts resulting in changes in shape and dimensions of the joints of the internal fastening structure and of the resonating plate. The enclosure is typically made of isotropic material such as ceramic, glass, steel or the like, or any combination of these materials. The piezoelectric material is always anisotropic. Accordingly, the temperature coefficients of expansion (TCE) of the piezoelectric resonator plate will differ from the TCE of the enclosure in at least one direction, resulting in strain/stress in the fastening structure which in turn is applied to piezoelectric resonator plate.
In the processes of manufacturing and testing and in applications the resonator may be subjected to significant temperature variations. Differences in TCE will result in different dimensional changes (deformations) of the enclosure and resonator plate. These differences are to some degree transferred between the enclosure and resonating plate through the internal fastening structure.
As used herein the term “mechanical strain” describes the stresses/strains due to external mechanical loads, resulting in changes in the shape and dimensions of the resonating plate and consequently in strain/stress of joints of the internal mounting structure. In many applications the resonator must maintain frequency stability under vibrations and/or significant external accelerations (G-shock). To this end, the internal fastening structure is preferably flexible to provide damping in a spectrum of vibrations to reduce mechanical strain induced by externally applied acceleration. The internal fastening structure is preferably strong enough to prevent its breakage under significant inertial loads due to G-shock. It can be seen, therefore, that the design of the internal fastening structure is a trade-off between mechanical strength and the transfer of minimum strain/stresses due to mechanical deformations of resonating plate.
As used herein the term “elastic recovery time” relates to the phenomenon of residual frequency shift, which has been observed after resonators have been tested under vibrations and after G-shock tests. The internal fastening system must include at least two electro-conductive joints connected electrically to external terminals and to two different electrodes plated onto the resonator plate. In other words, the resonator plate must be fastened inside enclosure at least with two joints. Generally two or more joints, spaced apart in some appropriate way, will result in thermal strain in the internal fastening structure if the TCE of the joined parts are different. Mechanical strain in the internal fastening structure is a result of differences in the deformations of joined parts under external loads. After an external load, which may be thermal or mechanical or both, is removed the internal fastening structure tends to relieve internal strain/stresses. The term “elastic recovery time” relates to how fast the elastically-deformed internal fastening structure restores its initial state or establishes a new shape/volume depending on the magnitude of the strain due to the load. The elastic recovery time generally depends on the elastic modulus of the fastening structure as a function of the magnitude of the strain.
Ideally, the elastic recovery time of the internal fastening structure should be equal to the elastic recovery time of both joined parts. If the elastic recovery times are different the joined parts will be strained after external, thermal or mechanical, loads are removed.
An absolutely flexible internal fastening structure would not introduce thermal and mechanical strain issues, but such a structure would not provide any mechanical support for a resonator plate. The internal fastening structure must include at least one elastic joint.
As used herein the term “interposer” relates to a material that is placed as an intermediary between the piezoelectric material and the enclosure.
Current resonator mounting techniques will now be described with reference to prior art resonators shown in FIG. 1 to 4. The arrangement in FIGS. 1 and 2 utilizes two joints as the internal fastening structure. As will be described below, this arrangement has the short side of a strip resonator plate fastened to electrodes inside the enclosure by means of two elastic joints made of electrically conductive glue or another form of suitable conductive adhesive.
The enclosure in this arrangement comprises a ceramic casing 1 with electrodes 4 connected to external terminals, a seal ring 2 and a welded stainless steel lid 3. A resonator plate 6 is fastened to the electrodes 4 by means of conductive flexible adhesive 5. The resonator plate 6 has conductive electrodes 7 on its top and bottom surfaces to excite electromechanical vibrations in the resonator plate by application of an electric field across the plate.
Although this arrangement affords simplicity of assembly, it does make it difficult to limit movement of the resonating plate relative to the enclosure under external acceleration.
In the arrangement of FIG. 3, the prior art resonator uses metal elastic holders as the internal fastening structure. The resonator plate 10 is glued to holders 11 made of thin metal and welded to conductive wire leads passing through the glass filled holes in a base 12. A can 13 then is welded to the base 12 to provide a hermetically sealed enclosure.
This arrangement benefits from reduced thermal deformations and increased mechanical strength. However there are strict requirements in the positioning of the joints relative to the crystallographic axes of the resonating plate. Furthermore, this design is difficult to miniaturize.
FIG. 4 shows a further prior art resonator arrangement. The resonator includes a ceramic casing 21 and a lid 23 as in FIG. 1. One joint 25, which is made of non-elastic strong adhesive, is placed in the middle of one of the short sides of resonating plate 26 and connects the bottom electrode of resonating plate 26 with one of the internal terminals 24. The top electrode of the resonator plate 26 is connected to the opposite internal terminal 24 with a flexible wire joint 27 in the middle of opposite short side of the resonating plate 26.
This design allows for low thermal deformations as both the enclosure and the resonating plate may expand independently. However the design is complex and there may be significant mechanical deformations in the resonator plate under vibrations and external acceleration due to the necessity to use a strong and rigid adhesive, as only one joint can be used resulting in cantilevered mounting of the resonating plate.
There have been other prior art techniques to mount a piezoelectric material within a package. U.S. Pat. No. 6,777,858 to Bang et al. relates to ceramic package for a crystal oscillator. The ceramic package includes a bottom sheet, to which a ceramic buffer sheet is attached, and a crystal wafer that is mounted on the buffer sheet. The buffer sheet serves to support to crystal wafer while protecting the wafer by absorbing external impact applied to the crystal wafer.
U.S. Pat. No. 5,250,870 to Fenlon et al. relates to a crystal package suitable for surface mounting. The piezoelectric device (crystal) is compliantly mounted to a thin package by dollops of adhesive. FIG. 1 of this patent shows the use of four adhesive dollops, while FIG. 2 shows the use of two adhesive dollops to mount the crystal.
A four-point mount for a quartz resonator for improved acceleration sensitivity is described in the 1992 IEEE Frequency Control Symposium paper by Larry D Clayton and Errol P Eernisse, entitled “Four-Point SC-Cut Crystal Unit Design Calculations for Reduced Acceleration Sensitivity”. This paper describes the use of an integral mount having four V-clips spaced at equal angular intervals around the circumference of an SC-cut crystal.
A quartz resonator with an internal fastening structure to improve acceleration sensitivity is described in the 1991 IEEE Forty-Fifth Annual Symposium on Frequency Control paper by Y. S. Zhou and H. F Tiersten, entitled “An Analysis of the Normal Acceleration Sensitivity of Contoured Quartz Resonators Stiffened by Identical Top and Bottom Quartz Cover Plates Supported by Clips”. This paper describes a biconvex quartz resonator that is sandwiched between identical quartz plates and attached by sidewalls. Despite an improved mechanical strength this prior art does not provide for compensation of residual frequency shifts due to possible changes in the relative position of resonating plate and the quartz plates if plastic deformation of the sidewalls takes place.
It is an object of the present invention to provide an improved resonator that overcomes or alleviates any of the above disadvantages or at least to provide the public with a useful choice.